Skip to main content

Full text of "Preliminary Note on Magnetic Circuits in Dynamo- and Magneto-Electric Machines"

See other formats

.292 Lord Elphinstone and C. W. Vincent. [Nov. 20, 

degree, the power of absorbing the ultra-violet rajs of the spectrum, 
though they are inferior in this respect to benzene and its derivatives, 
to which class of bodies they are so closely allied. 

2. Terpenes with the composition C 15 H 24 have a greatly increased 
absorptive power for the more refrangible rays, that is to say, they 
withstand dilution to a greater extent the greater the number of carbon 

o o 

atoms in the molecule. 

3. Neither the terpenes themselves nor the oxidised or hydrated 
derivatives occasion absorption bands under any circumstances when 
pure, but always transmit continuous spectra. 

4. Isomeric terpenes transmit spectra which generally differ from 
one another in length, or show variations on dilution. 

5. The process of diluting with alcohol enables the presence of 
bodies of the aromatic series to be detected in essential oils, and even 
in some cases the amount of these substances present may be estimated. 

' Several diagrams in illustration of the kind of absorption exerted 
'by the different substances are presented with the complete paper. 

III. " Preliminary Note on Magnetic Circuits in Dynamo- and 
MagDeto-Electric Machines.' 5 By Lord Elphinstone and 
Charles W. Vincent, F.R.S.E., F.C.S., F.I.C. Received 
July 26, 1879. 

The experiments which form the subject of the present note were 
.made in connexion with an investigation as to the best form for the 
construction of a dynamo -electric machine, intended to furnish cur- 
rents of high intensity in great quantity. The principle deduced 
.applies equally to magneto- electric machines. 

The source of power in all dynamo- electric machines being electro- 
magnets whose cores are already slightly magnetic, it appeared to us 
necessary to consider the conditions under which the initial force of 
such machines is best obtained. 

For this purpose we made use of a fj electro-magnet having a core 
of soft iron 2 inches in diameter and 36 inches long. The arms of the 
U were 4 inches apart. The exciting helices were two sheet copper 
reels, 12 inches long, fitting closely upon the uprights of the |J; ^ u ^ 
readily removable. Bach of these reels was coiled with 200 yards of 
"No. 14 double covered copper wire. 

Two cores of soft iron, of the same diameter, and each 12^ inches 
long, and which could be magnetised by the same helices, were also 

The principal armature was of soft iron, 8 inches in length, by 
.2 inches in width, and 1 inch thickness, rounded at the ends. Its face 

1879.] On Dynamo- and Magneto- Electric Machines. 293 

fitted approximately close to the poles of the \J magnet, whose faces it 
completely covered when placed upon them. 

Other armatures and magnets were employed, the form of which 
we propose to describe in a future paper. 

The iron of which the \J and the straight cores were made was 
found to be exceedingly plastic as regards molecular magnetic polarity. 
In a few seconds after the cessation of an electric current from twenty 
quart Bunsen cells acting through the above helices, they were in- 
capable of attracting and holding even fine iron filings. 

The U magnet tested with a suspended magnetic needle was found 
to retain some magnetic polarity after many days ; in fact, it is 
doubtful if the magnetism ever entirely disappeared, except when the 
core was subjected to special treatment. 

On the other hand, the straight cores lost their induced magnetism 
more rapidly, and when, having been demagnetized either by time or 
by the mode described further on, they were placed in the line of the 
magnetic dip, they showed poles in accordance therewith ; and on re- 
versing the position of the core, these poles were immediately reversed 
without its being necessary to resort to striking the bars or other 
means of putting them in a. state of vibration. It was thus demon- 
strated to our minds that if iron of similar quality, and in this form, 
were made use of for the electro-magnet cores in a dynamo-electric 
machine, the initial force producing the electric currents of the machine 
could not be due to residual magnetism, but rather to the lines of 
magnetic force of the earth. 

The current from four Buns en cells sent round the \J magnet fixed 
the armature so firmly that it could not be pulled, or even slid off, by 
the utmost exertion of one man's strength. 

On breaking battery contact, if both poles were completely covered, 
a direct pull failed to separate armature and magnet. The armature 
could, however, though with difficulty, be slid off ; the difficulty of 
movement greatly increasing as the edge of the poles was approached. 
For instance, on attempting to slide the armature off the north, the 
south, or both poles, the resistance became greater as the point of final 
communication between the poles through the moving armature was 
approached. This was found to be the case whatever time had passed 
between the rupture of contact and the first movement of the arma- 
ture. (Sometimes many days elapsed.) In very many experiments it 
was found, moreover, that, provided neither pole had been completely 
uncovered, on sliding back the armature to its normal position, the 
magnet, which with its stand and coils weighed over 58 lbs., could be 
lifted by it. 

A current from four Bunsen cells, almost momentary in duration, 
sufficiently magnetised the core to produce all the above effects. 

If, whilst the current flowed round the \J magnet, the armature 

294 Lord Elphinstone and 0. W. Vincent. [Nov. 20, 

rested on one pole only, it was of course strongly held ; but on break- 
ing contact it was at once set free, and fell off if not balanced ; the 
magnetism of the \J core immediately falling to its minimum, as shown 
by suspended test needles. If, however, the most minute point of 
connexion existed between the armature and the other pole, in addi- 
tion to its complete contact with the one it covered, it continued to 
be firmly held long after battery contact was broken. 

It being thought that possibly the effects described were partly duo 
to molecular attraction of the iron atoms when brought into close 
contact under magnetic stress, the poles were coated with a layer of 
tallow, but if this was sufficiently thin, the magnet could still be 
lifted by the armature after breaking battery contact. When the 
tallow was broken into small lumps, allowing light to be seen be- 
tween magnet and armature, the same result was obtained. 

Thus absolute metallic contact was found to be unnecessary for the 
retention of a considerable amount of magnetism by the \J core and its 
armature, when in magnetic circuit. With a piece of writing paper 
interposed between the poles and armature, they were held together 
with great force long after battery contact was broken ; but when the 
distance was increased by the interposition of cards, nails, or wires, 
to T T (T of an inch, the residual attractive force was very much lessened. 
When the magnetic circuit becomes more open the residual magnetism 
dies away in about the same proportion as the attractive force of core 
and armature, whilst under the influence of the battery current 
it becomes less when the distance between them is increased. 

Interposition of thick glazed note paper caused such a diminution 
of the residual magnetism that the magnet could no longer be lifted 
by the armature. 

The experiment was varied by putting lengths of fine silk thread 
straight across between the armature and the magnet ; in this, as in 
the former experiments, the armature was firmly held, and the magnet 
could be lifted by it. There was no point of actual metallic contact, 
and light could be seen over both magnetic fields, except at the thin 
lines where the silk threads were. The 581b. magnet, when lifted by 
the armature, was thus literally suspended in the air (like Mahomet's 
coffin) by the magnetism remaining in the almost closed circuit, and 
this long after the exciting electric current had ceased. (The experi- 
ments were made at intervals of four hours, twenty-four hours, three 
days, four days ; the armature had always ultimately to be wrenched 

The same result was obtained with plates or slips of zinc, copper, 
platinum, silver, and aluminium foils, gutta-percha tissue, em- 
broidery cotton, &c, and appeared to depend entirely on the distance 
between poles and armature, irrespective of the nature of the inter- 
posed body. 

1879.] On Dynamo- and Magneto-Electric Machines. 295 

When the straight cores were placed on the poles'of the \J magnet, 
and a current passed round the latter, attraction ceased the moment 
the battery contact was broken ; but if, while the current was passing, 
the armature was placed on the poles of the cores, the whole system 
was firmly held together, though, the current no longer flowed. 

There would appear to be no limit to the length of time during 
which the stored-up magnetic force exerts itself in such metallic 
circuits (closed, or nearly so) as are described above, for it was 
found that, after periods varying from one to fourteen days from the 
time of a momentary passage of an electric current round the cores, 
the attractive force was as great, or even greater, than at the first 

A small electro-magnet, |j -shaped, with limbs 6 inches long, having 
a core of f-inch iron, and helices consisting of 4 layers of No. 16 
covered copper wire, had for its armature a similar U core uncoiled. 
The uncoiled U was hung up, and the electro-magnet held beneath it, 
the poles of each being opposed : a current from four Bunsen cells 
was then sent through, the coils for a few seconds. Not only did the 
electro-magnet (weighing, with its coils, several pounds) remain 
firmly attached to its armature, but the hanging on to it subsequently 
of 8 pounds additional weight failed to detach it. 

A further proof of the large amount of magnetism held captive in a 
circuit thus closed was afforded by the following experiments. On 
connecting the ends of the wires from the helices with a galvanometer 
and resistance-coil, deflections varying from 40° to 90° were obtained 
with a resistance of 1,700 ohms in circuit each time the armature was 
forced away from the poles of the large \J magnet, after the passage 
of a current from four cells of a few seconds' duration. By careful 
manipulation, sparks between the ends of the helix wires were also 
obtainable each time the closed magnetic circuit was opened. (In one 
case a week had elapsed betwixt the passage of the current and 
obtaining of the spark.) 

In all these experiments, when the circuit was completely closed 
there was no external magnetism apparent, but on slightly breaking 
contact between the poles and armature, magnetic poles could be 
detected. Slight irregularities of the surfaces in contact likewise 
caused the development of poles. 

A heavy magnetic needle, 4 feet distant from the magnet, if 
deflected by the U magnet, uncovered by its armature, 45°, would fall 
back to 5° when the current ceased. If the poles were now covered 
by the armature, the needle went to 0°. Passage of the current from 
four cells would now give a deflection of about 38° ; on the current 
ceasing the needle would come back to 0°, and rise again to 5° on 
removal of the armature : but if, instead of immediately pulling off the 
armature, the two ends of the wires of the helices were connected 


On Dynamo- and Magneto- Electric Machines. [Nov. 20, 

together, and then the armature was forced off, the needle would 
swing 20° and fall back to 5° very slowly (in about fifteen minutes). 

If both poles of the U magnet were caused to be of the same name, 
and the armature placed upon them., there was no attraction after 
breaking battery contact. 

The straight cores, if placed upon the \J magnet connected by the 
armature, and then magnetised in such a way that the poles of the cores 
faced like poles of the {J magnet, retained no magnetic polarity when 
taken away from the system, whereas when they formed part of such 
a closed circuit as we have above described, the bars retained sufficient 
polarity to affect a magnetic needle for some time. 

We may here remark that the attractive force of electro-magnets 
for each other, in what we call open circuit, is not nearly as great as 
in a closed circuit. For instance, the \J magnet could not be lifted 
by the straight cores placed upon its poles, even with a current from 
six Bunsen cells running round the helices ; but on bridging the circuit 
with the armature the whole mass, weighing 82 lbs., could be raised 
from the ground with the current of only two cells, and quite irre- 

1879.] On the Transit of Venus. 297 

spective of the position of the exciting helices, whether both were on 
the magnet, both on the cores, or one on the magnet and one on a core. 

From the foregoing experiments it appears clear that the more near 
the approach to a closed magnetic circuit, the stronger is the field of 
force, and the longer is retained the magnetism of the mass or masses 
of iron constituting the circuit. The same rule holds good with 
regard to permanent magnets. • In closed circuits the attractive force 
is at its height, and diminishes in intensity as the magnetic field is 
more extended. But the parallel goes beyond this, for the more open 
the magnetic field, the more rapidly is the magnetic force itself dis- 

These principles have guided us in the construction of a dynamo- 
electric machine of whose magnetic circuits we here present a sketch, 
and which we hope to describe more fully in a future paper. 

In the accompanying diagram six fixed electro-magnets are shown, 
having alternate poles, opposite to which, and at a very short distance, 
are placed three other electro-magnets so arranged with opposing poles 
as to form three nearly closed circuits. Coils of wire are made to 
revolve so as to cross the intervals between these opposing poles, and 
the electric currents induced in the moving coils are made to pass 
round the electro-magnets. 

IV. " Further particulars of tbe Transit of Venus across the 
Sun, December 9, 1874 ; observed on the Himalava Moun- 
tains, Mussoorie, at Mary- Villa Station, Lat. 30° 28' N., 
Long. 78° 3' E., height above sea 6,765 feet, with the 
Royal Society's 5-inch Equatoreal." Note III. By J. B. 
N. Hennessey, F.R.S. Received October 4, 1879. 

1. The object of the present note is to add to Notes I and II* some 
particulars of the transit not detailed in those notes. The latter con- 
tained only sufficient extracts from my observatory notes in connexion 
chiefly with the three contacts which I observed; as, however, various 
other facts, besides the contacts, were developed in course of the 
transit, and elicited remarks from me at the time, it seems desirable 
that a complete transcript of these observatory notes should also be 
put on record ; both in connexion with what hereafter follows, and 
also to meet any possible future requirements of details, such as 
expressed by Captain Tupman in his discussion of the mean solar 

* See " Proc. Koy. Soc." Vol. xxiii, pp. 254, 379. 

f Royal Astronomical Society, "Monthly Notices." Vol. xxxviii, p. 452.